DEVELOPMENT LIMITATIONS
Physical geography has always been an important and many times a controlling factor in the determination of suitable locations for developed land uses. The elements of terrain, soil conditions, surficial geology, surface water bodies and drainage patterns, and groundwater are included among the important developmental factors that need to be considered. The effects that the characteristics of these elements have upon man and his development range from potential dangers to life and property to just plain nuisances. Areas that pose hazards to human life and property naturally must be avoided or the characteristics must be altered if possible.
While man has increased his ability to alter and shape the natural landscape and is able to alleviate many of the physical problems he encounters, he does so at a cost – sometimes measured only in financial terms, other times in both financial and environmental terms. Development in harmony with the characteristics of the natural environment offers both financial and environmental benefits. Historically, development patterns have not reflected total consistency with natural environmental characteristics. The annual loss of life and property through flooding in many parts of the United States attests to this fact. Where development has been contrary to environmental characteristics it has often been due to shortsightedness and lack of knowledge of the long-range implications of actions. The results of such actions often involve long-term social and environmental losses. This section concerns development limitations within the County that must be considered to avoid such losses in planning for future development.
To simplify the analysis and presentation of information, individual development factors have been considered as they apply to four general limitation categories: floodplains, building site limitation, septic tank filter field limitations, and landslide areas. These four general categories cover all of the physical elements previously mentioned as important developmental factors. Among the factors considered within the building site limitations category are land slope and soil characteristics such as bearing strength, shrink-swell potential, composition and depth to bedrock. Soil characteristics such as permeability and groundwater table are considered in the septic tank filter field limitations category. Factors considered in the other two categories are self-explanatory.
The State Geology Department has completed a study of the geologic hazards of four selected areas of Marion County.4 These areas are where the more significant geologic hazards exist and where present and future development might cause problems. They are the South Salem Hills, East Salem-Aumsville area, Abiqua Creek area, and the North Santiam and Little North Fork of the Santiam area.
The areas of floodplain, landslide and steep slope hazards are shown on the map “Development Limitations.” The areas of building site and septic tank limitations are defined by the general soils associations shown on the general soils map in this report. Tables 4, 5, and 6, taken from the geologic hazards report, provide general guidance in reviewing land use activities for potential problems. These tables indicate the level of concern for activities proposed in identified hazard areas.
It should be emphasized that this information is not for detailed site evaluation. This information should be used to identify potential problem areas relative to proposed development. More site-specific information will need to be included in each proposal for development. The following paragraphs analyze and describe the nature and extent of the problems of development limitations in Marion County.
Floodplains
Historically, waterways have been attractors of growth. The level land adjacent to streams and rivers provided opportunities for agricultural production while the waterways themselves provided a source of water supply and transportation. Urban centers naturally developed in such locations. However, the choice of such sites many times resulted in a conflict between man and nature. When called upon to carry greater than normal quantities of water, streams and rivers occasionally inundate the comparatively flat areas immediately adjacent to the normal stream channels. In actuality, these areas are only extensions of the normal stream channels and are commonly referred to as floodplains. Conflicts inevitably arise when urban centers or other developed uses are constructed in a floodplain. Flooding of such areas involves a hazard to both life and property.
In Marion County, several urban centers have encroached upon the floodplain. The largest amount of urban encroachment is present in the Salem urban area. As a consequence of this encroachment, the floods of the 1964 season resulted in property damage and losses of $8,482,000 in the Salem area.5 Flooding necessitated the evacuation of Salem Memorial Hospital and affected many parts of the city and surrounding suburbs. More than 36 separate areas experienced localized flood damage. Floodwaters were nine feet deep in Keizer and two feet deep in West Salem. Backwaters from the Willamette River prevented runoff through Shelton Ditch, Pringle Creek, Claggett Creek and other discharge streams, creating further damage.
Within the last 50 years, attempts have been made to reduce flood damages in this nation through structural control of the floodwaters, i.e. by keeping the floodwaters away from man. Such projects as dams and reservoirs, levees, revetments and channel improvements have been used for this purpose. In addition, flood forecasting, evacuation and floodproofing have also been use to reduce flood damages. While all of these efforts unquestionably have been of value in reducing flood losses, the fact remains that flood damages have continued to rise because of increased development in the floodplains. This indicates that control of floodwaters is not the total solution to flood damage reduction. Increasing recognition is being given to the need to keep man away from the floodwaters. Quite logically, there would be no more conflicts if this were to be accomplished. However, it is recognized that total accomplishment of such an objective is probably unrealistic as it would place severe restrictions on many of man’s activities. The current approach to floodplain management combines structural control measures with land use regulation. Such management is designed not only to reduce flood damages but to safely permit certain uses of the floodplain.
Flood control structures are primarily the realm of the U.S. Army Corps of Engineers, since their costs and scope are beyond the capabilities of local or State government. Land use controls are, on the other hand, best accomplished at the local level through city and County zoning ordinances. The major impetus to the adoption of local control ordinances has come from the enactment of the National Flood Insurance Program by the U.S. Congress in 1968. This program, administered by the Federal Insurance Administration in cooperation with the private insurance industry, made insurance against flood caused losses available to residences of participating jurisdictions. In 1973, the program was amended to make flood insurance mandatory in flood hazard areas by requiring Federal agencies and Federally backed lenders to require flood insurance for all projects and loans. In return for making low cost flood insurance available to existing development in floodplains, the County is required to enact land use control measures to minimize flood damage to new development. This process involves enactment of County floodplain development control ordinances specifying activities allowable in the identified flood prone areas. To accomplish this, it is necessary to identify specifically the area of potential inundation and the expected flood height. The floodplain area applicable to the insurance program and local land use control is that area in which a flood may occur on a one percent chance in any one year.
Marion County entered into the preliminary flood insurance program by adopting a floodplain ordinance in 1974 that controlled development in the 100-year floodplain of the Willamette River, Santiam River and parts of their tributaries.
To effectively accomplish the program, the FIA developed floodplain delineation maps and reports with assistance from the U.S. Army Corps of Engineers. By performing analysis of the hydrological characteristics of all flood prone areas, detailed floodplain maps of Marion County were produced showing the location of the floodway and floodway fringe in addition to the flood elevation on special areas. Detailed studies were done of areas where development is more likely to occur while approximate information was presented in most rural resource lands. When this data was completed, Marion County was then placed into the final or permanent flood insurance program in August 1979. This required the revision and updating of the County land use control ordinance contained in the Zoning Ordinance. The FIA floodplain maps provide the basis for defining the area and elevation data to implement the County ordinances.
The following diagram shows the various floodplain characteristic areas used on the flood maps:
Figure 3
Within the total land area inundated by a flood, the degree of hazard involved may vary considerably from place to place. Some areas are subject to the movement of a large volume of high velocity waters, while others are merely subject to the storage of relatively shallow, slow moving waters. For the purposes of understanding the different nature of areas within a flood, the channel and adjacent floodplain that is needed to adequately discharge the waters of the 100-year flood. It is within this area that the major volume of floodwater is discharged. Water depth and velocity are usually both relatively high. Open land uses such as agriculture are the best uses of this area so as not to impede the floodwaters. The floodway fringe is that portion of the floodplain lying outside of the floodway but within the flood limits. Structures erected within this area do not create significant hazards. Structures constructed in this area are required to have floor elevations above the flood level or be floodproofed. Urban or concentrated development is not appropriate in this area.
A hydrological study of the areas of special concern for their flood damage potential was done by engineers contracted to the FIA. Where a detailed study was not made, the general data supplied by the Corps of Engineers is used.
The program allows the more shallow flood fringe areas to be developed, built upon and filled provided any development is above the calculated flood elevation when confined within the floodway (Line C – D in Figure 3).
Those areas shown as being subject to flooding are shown on the Development Limitations Map that generally indicates the limits of the 100-year flood level. The more specific floodplain data is shown on the Federal Insurance Administration floodplain maps on file in the Marion County Planning Department.
To give full perspective to the flooding issue it is important to point out that a flood larger than the 100-year level may occur. A 500-year flood is the most severe flood that could occur, resulting from a simultaneous occurrence of the most critical meteorological conditions. These are extremely rare and are not accounted for in the flood protection program. It should also be pointed out that major floods of an infrequent probability may occur in two or more consecutive years and more than one major flood may occur in any one year. An example of the latter case is the December 1964 and January 1965 floods in Marion County and the Willamette Valley, both of which were major floods. It also needs to be stressed that the floodplain is subject to constant change due to modifications in the topographic pattern of land, modifications of the drainage pattern, urban developments which cause added runoff and construction of water storage projects. Consequently, it is not possible to predict the exact limits of inundation during future floods. However, precautions against damage of the 100-year flood will sufficiently protect the vast majority of the floodplain development.
Landslide Areas
Areas of landslide activity or unstable slopes are usually unsuitable for development because of hazards to human life and property from earth movement. The areas within the County identified by the State Engineering Geologist as active or inactive landslide areas are shown on the Development Limitations Map. A major active landslide area is located on the west-facing slope of the Salem Hills. The slides in this area have developed on steep slopes of soils originating from the marine sedimentary bedrock units. Landslides also occur in the canyon of Abiqua Creek about five miles east of Silverton and along the slopes of the Little North Fork of the Santiam River. In these areas, the slides are developed in deeply weathered tuffs of the Mehama Volcanics. Landslides may also occur in the clay soils overlying the Columbia River Basalt in the Salem Hills area and in the Waldo Hill Silverton Hills area, if slopes are artificially over-steepened.
Steep slopes associated with landslide activity areas are themselves a deterrent to high-density development. But such areas of steep and unstable slopes may be attractive to low density residential development because they have a view or because they possess other site amenities. In any case, development in any identified active or inactive landslide area should be reviewed on an individual site basis. Special engineering geology studies may be required to determine if proposed development can be safely accommodated.
Building Site Limitations
As mentioned previously, among the factors that should be considered when determining limitations for building or development are land slope and soil characteristics such as bearing strength, shrink-swell potential, composition, and depth to bedrock. A high groundwater table may also involve certain construction difficulties. The foregoing limiting factors, and others, have been used by the Soil Conservation Service (in cooperation with the Oregon Agricultural Experiment Station) in analyzing and rating soil limitations for various uses.
The classification system used by SCS involves the rating of a soil association for a particular use by degree of limitation for that use: slight limitation, moderate limitation, or severe limitation. Slight limitations either do not require any special planning, design, or management, or the restrictions are easily overcome. Moderate limitations have restrictions that can be overcome with planning, careful design, and good management. Severe limitations indicate that this use is doubtful and generally unsound.6 It should be understood that these ratings and their application to soil associations, which generalize the detailed soil units, are suitable only for large-scale general planning purposes. Only limited consideration has been given to possible corrective measures which might be employed to alleviate the limitations noted, the design criteria for such corrective action and, in general, the economic feasibility of undertaking such corrective action. More detailed information on each soil type is included in the Marion County Soils Survey.
All 23 soil associations in Marion County are listed and rated for degree of building site use limitations in Table No. 30. The limiting factors that cause a particular association to have a rated limitation are listed beneath the table. The percentage figures listed next to the ratings indicate the amount of land area within the soil association that has that particular degree of limitation. For example, 40 percent of the land area within the Chehalis Cloquato soil association has only slight limitation for building site use. However, 60 percent of the association area is rated as having a severe limitation for building site use. The limiting factors (8 and 9) causing the severe rating are a high fluctuating water table during winter and early spring and flood hazard.
The location of these soil associations is shown on the General Soils Map.
Many of the soil associations occurring on the alluvial terraces are treated as having severe limitations for building site use because of a seasonally high groundwater table. This factor requires special precautions to protect basements of houses and buildings from water infiltration. Either basement floor slabs and walls must be designed to withstand hydrostatic uplift pressures or water levels must be controlled. Hydrostatic uplift can also damage swimming pools unless water is retained in the pools during the high groundwater season or unless the structure is designed to withstand uplift pressure. Another situation that should be anticipated in these areas is the collection of groundwater in shallow extractions. In such instances, special precautions may be required to protect the excavation slopes.
Some soil associations, again principally on the alluvial terraces, have limiting factors related to soil stability and the ability to bear the loads of buildings. A high shrink-swell potential is a problem in the Bashaw, Concord-Dayton-Amity, Santiam, Steiwer-Chehulpum-Hazelair, and McCully Associations. The last two associations are not on the alluvial terraces, but are low foothill associations. In addition to a high shrink-swell potential, these two foothill associations also have soils within them that possess poor stability. Low shear strength is an additional problem in the Steiwer-Chehulpum-Hazelair and Wapto-Waldo associations. Soils possessing any of the foregoing characteristics will obviously present special problems to providing adequate building foundations. Required solutions may prove to be economically infeasible.
The degree of hazard to life and property resulting from the foregoing building site limitation factors is obviously less than the degree of hazard related to flooding. Generally, the hazard involved relates to property damage or increased building costs. In normal construction practice, many of the limiting factors cited are taken into account and corrected. This should be apparent from the amount of developed land occupying areas rated severe – much of the Salem area has a seasonally high groundwater table as a building site limitation factor.
|
|
Soil Association |
Building Sites |
Limiting Factors |
Septic Tank Filter Fields |
Limiting Factors |
|
1. |
Chehalis-Cloquato |
Slight 40% |
– |
Slight 40%* |
– |
|
Severe 60% |
8, 9 |
Severe 60%* |
8, 9 |
||
|
2. |
Cloquato-Newberg |
Slight 5% |
– |
Slight 5% |
– |
|
Severe 95% |
9 |
Severe 95%* |
9 |
||
|
3. |
Cloquato-Newberg-Camas |
Slight 90% |
– |
Slight 85% |
– |
|
Severe 10% |
n.g. |
Slight 15% |
n.g. |
||
|
4. |
Bashaw |
Severe 100% |
7, 34 |
Severe 100%* |
8, 13 |
|
5. |
McAlpin-Abiqua |
Moderate 95% |
8, 34 |
|
|
|
Severe 5% |
n.g. |
Severe 100%* |
8, 12 |
||
|
6. |
Wapato-Waldo |
Moderate 5% |
n.g. |
|
|
|
Severe 95% |
7, 9, 36 |
Severe 100%* |
7, 8, 9, 13 |
||
|
7. |
Semiahmoo-Labish |
Moderate 15% |
n.g. |
Moderate 15% |
n.g. |
|
Severe 85% |
7, 8, 9 |
Severe 85% |
7, 8, 9 |
||
|
8. |
Concord-Dayton-Amity |
Moderate 10% |
n.g. |
Moderate 5% |
n.g. |
|
Severe 90% |
6, 7, 8, 34 |
Severe 95% |
6, 7, 8, 12 |
||
|
9. |
Amity |
Severe 100% |
6, 8 |
Severe 100% |
6, 8, 12 |
|
10. |
Woodburn |
Slight 10% |
– |
Slight 10% |
– |
|
Moderate 75% |
1, 8 |
|
|
||
|
Severe 15% |
n.g. |
Severe 90% |
1, 8, 13 |
||
|
11. |
Clackamas |
Slight 25% |
– |
Slight 25% |
– |
|
Moderate 65% |
6, 8 |
|
|
||
|
Severe 10% |
n.g. |
Severe 75%* |
8, 12 |
||
|
12. |
Sifton-Salem |
Slight 90% |
– |
Slight 90%* |
– |
|
Moderate 5% |
n.g. |
Severe 10%* |
n.g. |
||
|
13. |
Courtney-Clackamas |
Slight 35% |
– |
Slight 35% |
– |
|
Moderate 15% |
6, 8 |
|
|
||
|
Severe 50% |
7 |
Severe 65%* |
8, 12, 13 |
||
|
14. |
Santiam |
Moderate 80% |
1, 8, 34 |
|
|
|
Severe 20% |
n.g. |
Severe 100% |
1, 8, 13 |
||
|
15. |
Steiwer-Chehulpum-Hazelair |
Moderate |
1, 2, 3, 25, 35 |
|
|
|
Severe 50% |
4, 8, 13, 34, 36 |
Severe 100% |
1, 8, 12, 13, 25 |
||
|
16. |
Jory-Nekia-Salkum |
Slight 35% |
|
|
|
|
Moderate 55% |
1, 2, 23 |
|
|
||
|
Severe 10% |
1, 3, 23 |
Severe 100% |
1, 12, 13, 23 |
||
|
17. |
Nekia (gentle to strong slope) |
Moderate 100% |
1, 15, 23 |
Severe 100% |
1, 12, 23 |
|
18. |
Nekia (gentle to steep slope) |
Moderate 50% |
1, 15, 23 |
|
|
|
Severe 50% |
1, 23 |
Severe 100% |
1, 12, 23 |
||
|
19. |
McCully |
Slight 30% |
|
|
|
|
Moderate 10% |
1, 15 |
|
|
||
|
Severe 60% |
|
Severe 100% |
1, 8, 12, 13 |
||
|
20. |
Hullt-McCully |
Moderate 15% |
1, 33 |
|
|
|
Severe 85% |
1 |
Severe 100% |
1, 12 |
||
|
21. |
Kinney |
Slt.-Mod. 30% |
1, 15 |
|
|
|
Severe 70% |
1, 4 |
Severe 100% |
1, 11 |
||
|
22. |
Horeb |
Slt-.Mod. 75% |
1 |
|
|
|
Severe 25% |
1 |
Mod.-Severe 100% |
1, 11 |
||
|
23. |
Whetstone-Henline |
Severe 100% |
1, 23 |
Severe 100% |
1, 23 |
|
n.g. Not Given |
|||||
|
* Pollution to water supplies is a potential hazard |
|||||
|
1. |
Excessive slope |
|
2. |
Moderate erosion hazard |
|
3. |
High erosion hazard |
|
4. |
High slide hazard |
|
5. |
Moderately well drained |
|
6. |
Somewhat poorly drained |
|
7. |
Poorly drained |
|
8. |
High fluctuating water table during winter and early spring |
|
9. |
Subject to flooding |
|
10. |
Rapid or moderately rapid permeability (2.0 to 20 inches/hour) |
|
11. |
Moderate permeability (0.63 to 2.0 inches/hour) |
|
12. |
Moderately slow permeability (0.20 to 0.63 inches/hour) |
|
13. |
Slow or very slow permeability (less than 0.2 inches/hour) |
|
14. |
Gravel throughout soil |
|
15. |
Stony or cobbly in surface layer or subsoil |
|
16. |
Very gravelly substream at 20 to 40 inches |
|
17. |
Silt loam surface layer or subsoil |
|
18. |
Silty clay loam or clay loam surface layer or subsoil |
|
19. |
Clay surface layer or subsoil |
|
20. |
High organic matter |
|
21. |
Bedrock at less than 20 inches |
|
22. |
Basalt bedrock at 40 to 100 inches |
|
23. |
Basalt bedrock at 20 to 40 inches |
|
24. |
Sedimentary rock at 40 to 100 inches |
|
25. |
Sedimentary rock at 20 to 40 inches |
|
26. |
Pervious compacted permeability |
|
27. |
Poor compaction characteristics |
|
28. |
Limited supply of suitable material |
|
29. |
Thick overburden |
|
30. |
Excessive fines |
|
31. |
A4 or A5 AASHO engineering classification |
|
32. |
A6 or A7 AASHO engineering classification |
|
33. |
Moderate shrink-swell potential |
|
34. |
High shrink-swell potential |
|
35. |
Poor stability |
|
36. |
Low shear strength |
Septic Tank Filter Field Limitations
Sewage disposal by means of a septic tank and filter field system is one alternative that may be used when no public sewage collection and treatment system is available. This method of subsurface sewage disposal has been common practice in rural and suburban areas of Marion County for a number of years, as it has been in many other areas of Oregon and the United States. As the name implies, there are two major functional parts of the system: the septic tank and filter field. The tank receives the sewage and other wastes through a sewer line from the house (or other source), the solids settle within the tank, the liquid sewage (effluent) overflows into a drain-tile field, and through perforations in pipe the effluent is released to the surrounding soil where it is absorbed and filtered. The septic tank maintains an anaerobic environment (without oxygen) in which bacterial action takes place to digest the solids. However, within the drain-tile and filter field just the opposite condition is required. An aerobic environment (with oxygen) is necessary for proper operation of this part of the system. Oxidation of the effluent and treatment by anaerobic bacteria is necessary in addition to the filtering of suspended solids as the effluent passes through the soil. In the absence of oxygen within the soil, a rich black matter (ferrous sulfide) may form around the drain-tiles and clog them.7
Because this system of sewage disposal utilizes the soil as an integral part of the system, the characteristics of the soil determine whether or not this system can be used. Soil suitability for subsurface sewage disposal depends largely on the absorptive ability, or permeability, of the soil. But there are several other soil characteristics that may affect soil suitability, such as groundwater level, depth of soil, types of underlying material, slope of the land surface, proximity to streams or lakes, and flooding.
Permeability is the rate of water movement through the soil. Naturally, the greater the porosity of the soil, the faster the rate of water movement and the greater the permeability. Soil permeability is many times referred to in general terms such as slow, moderate or rapid; but it may also be measured in terms of the amount of water which will percolate through the soil over a given period of time. To be suitable as an effluent filtering medium, a soil must be within a certain minimum and maximum permeability range. The absorptive quality of the soil must be great enough to handle the volume of effluent discharged to it but not so great that the soil will not filter the effluent. Several publications give a suggested range of percolation rates that can be used when determining the suitability of a soil for filter field use.
A high groundwater table (temporary, seasonal, or year-round) within an area can render a soil unsuitable for filter field use. If the water table reaches the level of the drain-tile, the sewage effluent will be forced upward to the soil surface. This creates a health hazard, and under these circumstances the filter field obviously cannot operate properly. Even if the water table does not rise to this height, it must remain sufficiently low to prevent the effluent from reaching the groundwater. The State of Oregon regulations governing septic tank usage indicate that, “A temporarily perched water table shall not come in contact with the absorption facilities effective sidewall during any season of the year.”8 This means that the water table should not be closer to the ground surface than four feet for prolonged periods during the year.
In order to provide adequate soil depth for the filtration and purification of septic tank effluent, rock formations should be at least four feet below the bottoms of the trenches of the filter field.
Slopes of less than 12 percent usually do not create serious problems in either the construction or maintenance of filter fields, provided the soils are otherwise satisfactory. On steeper slopes, trench filter fields are more difficult to lay out and construct. In addition, there may be a serious problem in controlling the lateral flow of the effluent to the downhill soil surface. This downhill flow may reach the soil surface before the effluent is properly filtered, creating a potential health hazard. The problem of controlling lateral flow on steep slopes may be compounded if a layer of dense clay, rock, or other impervious material is encountered. If exposed on the hillside, the impervious layer will channel the unfiltered effluent to the ground surface downslope from the filter field.
Oregon law requires that filter fields be located at least 100 feet from any stream, open ditch, lake, or other watercourse into which unfiltered and contaminated effluent can escape and spread. Even with this limited precaution, it should be recognized that the natural filtering and treating effect of the soil cannot be depended upon to remove all bacteria from polluted groundwater, and extreme care must be exercised in locating a filter field in any area that is a potential source of water supply.
The Soil Conservation Service ratings of soil association limitations for septic tank filter field use and the limiting factors considered in their analysis are included in Table No. 7. This is the table containing the information on building site limitations discussed in a previous section. As with the building site limitation information, only the soil association with at least 50 percent of their area rated as severe have been mapped on the development limitations map. Likewise, two categories of limitation based on area coverage within the association have been shown. Unlike the information on building site limitations, the information on septic tank filter field limitation indicates that almost all of the soil associations have at least 50 percent of their area rated severe, and most of them have over 75 percent of their land area rated severe.
Limiting factors for filter field operation within the soil associations of the alluvial bottomlands are generally a flooding potential and a seasonally high groundwater table. On the alluvial terraces, the soil associations are also limited by a high groundwater table. In addition, many of the soils in level areas of the terraces (such as those within the Concord-Dayton-Amity association) are poorly drained and have slow permeability. In the soil associations of the low foothills, the limiting factors are generally excessive slope, inadequate permeability, or insufficient soil depth. Limiting factors in the mountain footslope soils are generally excessive slope and insufficient soil depth.
Considering other limiting factors for septic filter field use reduces the list of suitable soils even further. Of the 25 soils with suitable permeability, seven are definitely unsuitable for septic filter field use because of steep slopes. Another seven soils are of questionable suitability because of slope limitations.
It should be noted again that the soil association information is quite generalized and is suitable only for the most general, areawide use. There may be isolated areas within the affected association which do not share the development limitations; also, corrective actions may be possible in some instances. However, past experience in the Salem area (and other communities within the County) indicates that concentrated or suburban density development using septic tanks is only a short-term solution to the problem of sewage disposal. Maintenance of proper sanitation standards and good land use planning practice argue for the provision of public sewer services to areas of urban expansion prior to or simultaneously with their development.
Low-density (acreage) residential uses may be accommodated in many parts of the rural region by use of septic systems; however, only in limited numbers. Based on an analysis of the limitations within different physiographic areas of the County, it appears the low foothills have the fewest limitations for low-density residential use.
The potential pollution of ground and surface water from septic tank malfunction is a significant concern of the water quality planning program. The 208 Waste Treatment Management Plan being developed by the Mid-Willamette Valley Council of Governments will identify pollution problems relating to septic tanks and, hopefully, develop management proposals to minimize the problems.
4
Geologic Restraints to Development in Selected areas of Marion County Oregon, Herb Schlicker, Engineering Geologist, Oregon Department of Geology and Mineral Industries, 1977.
5
“Postflood Report, December 1964 – January 1965 Flood,” U.S. Army Corps of Engineers, Portland District.
6
Soils Survey of Marion County Area, Oregon, United States Department of Agriculture, Soil Conservation Service, in cooperation with Oregon Agricultural Experiment Station, September, 1972.
7
Economic Analysis of Sewage Control for Residential Suburbs, August, 1966, Carter W. Harrison, Stanford University, Palo Alto, California.
8
Regulations Governing the Subsurface Disposal of Sewage, 1978, Department of Environmental Quality, page 29, 71030(1)(d).